This isn't quite true. Support for legacy extensions is disabled by default, but can be re-enabled via the `extensions.legacy.enabled` flag in about:config.

Don't expect it to work very well, though, given that a lot of the supporting code for legacy addons was already removed from Gecko 57.

Huh, thanks for mentioning that! I didn't realise there was still a flag for it - serves me right for not doing my research.

Though losing Electrolysis will defeat one of the main benefits of running Firefox 57...

Though you lose multi-process operation, I think....

only on incompatible MP plugins.

however I've just ran nightly and although it says vimfx will work as a legacy plugin it's not working, so the deal's off. I shall not use my mouse!

You can try vimium-ff (FF 57-compatible). There is also Tridactyl [1] currently in development.

1: https://github.com/cmcaine/tridactyl

Only in Nightly.